Branched infinitely-variable hydromechanical transmission for motor vehicles

ABSTRACT

A branched, infinitely variable hydromechanical transmission with multiple power-transmission paths for motor vehicles has an expanded first shift range to facilitate broadening of the overall gear-shift range to achieve higher power density and eliminates the need for a starting clutch. The transmission includes a first hydrostatic unit with an adjustable volume, a second hydrostatic unit, preferably with a constant volume, and one or more summation gear trains for adding together the power transmitted from the transmission input along hydraulic and mechanical power-transmission paths. The input shaft of the transmission is connected directly to the drive motor.

This is a continuation of copending application Ser. No. 07/494,856filed on Mar. 14, 1990, which is a continuation of Ser. No. 07/300,175filed Jan. 17, 1989, which is a continuation of Ser. No. 07/113,187filed Jul. 16, 1987 now all abandoned.

The present invention relates to infinitely variable hydromechanicaltransmission with multiple power-transmission paths and having a firsthydrostatic unit with an adjustable volume and a second hydrostaticunit, preferably with a constant volume, and one or more summation geartrains for adding together the power which originally is separated atthe transmission input for transmission hydraulic and along separatemechanical power-transmission paths.

German Patent Specification 31 47 447 discloses a hydromechanicaltransmission with multiple power-transmission paths and having aplurality of gear-shift ranges which, similarly to the transmission ofthe invention, has a first hydrostatic unit with an adjustable volumeand a second hydrostatic unit with a constant volume in this knowntransmission, during start-up and with the first driving range clutchclosed, the first adjustable hydrostatic unit is adjusted to its maximumoperating volume and the summation planetary gear train is so designedthat in this operating state the transmission power output shaft isdriven at a predetermined output speed, which corresponds to the designminimum speed of the vehicle. However, this known transmission does notinclude either a bypass valve the high-pressure and low-pressure linesof the hydrostatic circuit to facilitate starting or a first drivingrange clutch which is arranged for used also as a starting clutch. Thisknown transmission includes a separate contrifugal starting clutch whichis connected between the drive motor and the input shaft of thetransmission. Such starting mechanism is very expensive and it is wellknown that centrifugal are not capable of gradual shock free engagementas is ordinarily required in passenger cars.

The object of the present invention is therefore to provide ahydromechanical transmission with multiple power-transmission pathswhich, in order to broaden the gear-shift range and/or to increase thepower density, has an expanded first driving range and does not requirea separate starting clutch.

Compared to the prior art transmissions of this type which includes anadditional starting clutch, the invention has the advantage that it issimpler and less expensive and conserves the space which would otherwisebe required for the starting clutch. Moreover, in accordance with thepresent invention the input shaft of the transmission maybe connecteddirectly to the drive motor.

Certain preferred specific embodiments of the invention will now bediscussed hereinbelow with reference to the drawings, in which:

FIG. 1 is a schematic illustration of a hydromechanical transmissionthat has multiple power-transmission paths and two hydro-mechanicalforward driving ranges, the first range being expanded by thetransmission-ratio dimension X, and the reverse range being similarlyexpanded,

FIG. 2 is a speed diagram for the specific embodiment of FIG. 1,

FIG. 3 is a schematic illustration of a transmission design that has twoforward driving ranges and one reverse range, the first forward drivingrange and the reverse range being expanded in the same way as in theembodiment of FIG. 1,

FIG. 4 is a speed diagram for the specific embodiment shown in FIG. 3,

FIG. 5 is a schematic illustration of a transmission design that hasthree forward ranges and one reverse range, the first forward range andthe reverse range being expanded and,

FIG. 6 is a speed diagram for the specific embodiment depicted in FIG.5.

A hydromechanical transmission system 100 with multiplepower-transmission paths which embodies the concepts and principles ofthe invention is illustrated in FIG. 1. Transmission system 100 includesa first hydrostatic unit A with an adjustable volume and a secondhydrostatic unit B which preferably has a constant volume. Transmissionsystem 100 also includes a four-shaft summation planetary gear train 4and a three-shaft planetary gear train 9 that is collocated therewithand preferably includes a planet gear on a carrier 20, a sun gear 21 andan internal gear 22. Transmission 100 also has three range namelyclutches, clutch 24 for the first forward driving range, clutch 25 forthe second forward driving range and clutch 23 for the reverse range.The summation gear train 4 comprises two individual planetary gearstages, each with one sun gear (14, 17), one planet gear carrier (15,19), and one internal gear (16, 18).

The first shaft of summation gear train 4 is connected to the planetcarrier 15 of the first planetary gear stage and to the internal gear 18of the second planetary gear step stage and has a direct connection tothe input shaft 2 which also provides input power directly to the firsthydrostatic unit A. The second shaft of summation gear train isconnected directly to the internal gear 16 of the first planetary stage,to the planet carrier 19 of the second planetary stage, to the sun gear21 of planetary gear train 9 and to a coupling element of the firstdriving range clutch 24. The third shaft 35 of gear train 4 is connecteddirectly to the internal gear 17 of the second planetary stage. Thefourth shaft of summation gear train 4 forms a sun gear 14 of the firstplanetary stage and is directly connected to a coupling element of thesecond driving range clutch 25. In this specific embodiment, the twohydrostatic units A and B may be compactly combined to present a singletransmission unit and/or a single constructional unit. Likewise, thesummation planetary gear train 4, the second planetary gear train 9 andthe set of clutches 23, 24, 25 may be arranged in separate modular form.As shown, the separate modular constructional units may be mountedcoaxially, as well as staggered parallel, relative to one another,depending on the vehicle requirements. The individual shafts may beconnected to each other in the staggered method of construction viasuitable ratio steps without changing basic the principles of thesystem. The staggered arrangement is of particular advantage intransmissions for vehicles with an engine that is installedperpendicularly, e.g., in passenger cars as well as in transmissions forheavy vehicles, buses, commercial vehicles, and absorbing machines.

The operation of the transmission is as follows.

As mentioned above, the summation gear train 4 is so designed thatduring start-up the second hydrostatic unit B is driven via the thirdshaft which carries the sun gear 17 of the summation gear train, whenthe first range clutch 24 is closed. The unit B is driven at a speedsubstantially higher than that of the first hydrostatic unit A when theoperating volume of the latter is set at its maximum valve. Under theseoperating conditions, the second hydrostatic unit B operates as a pump.According to the invention, the increased delivery of the secondhydrostatic unit B occurring during these operating conditions, whichcannot be accommodated by the first hydrostatic unit A, is equalized byopening of the bypass value 12 connected between the two high-pressureand low-pressure lines 13 of the hydrostatic circuit.

During start-up, the bypass valve 12 is gradually closed by a signal S,so that inevitably both hydrostatic units A and B, in accordance withtheir volumes, which, under these conditions are at their maximumvalves, are forced to reach the same speed which corresponds to aminimum travelling speed corresponding to the transmission-ratio point X(FIG. 2). This starting process corresponds to the closing process of astarting clutch of a conventional transmission or to the startingprocess of the prior art hydromechanical transmission system referred toabove. In order to obtain a gradual start-up without shock, the bypassvalve 12 is controlled by a signal S from the braking system, e.g. byrelease of the brake pedal, and/or by the engine speed and/or by aload-dependent signal, e.g., the operating pressures within thehydrostatic transmission A, B.

The starting mechanism of the transmission 200 depicted in FIG. 3 isidentical to that of transmission 100 of FIG. 1. However, transmission200 is distinguished by a different transmission system with multiplepower-transmission paths, which is based on the provision of twosummation planetary gear trains 5 and 6. The first summation planetarygear train 5 comprises a three-shaft planetary gear train which includesa sun gear 36, a planet carrier 37 for a planet gear and an internalgear 38. The second summation planetary gear train 6 is constructed withtwo planetary gear train stages. In this arrangement, sun gear 39 of onestage is constantly connected to sun gear 36 of gear train 5 and sungear 44 of the other stage is constantly connected to planet carrier 37of gear train 5. An output shaft planet gear carrier 43 of the secondsummation planetary gear train 6 is connected to another planetary geartrain 10 and to a coupling element 48 of the first range clutch 27. Acoupling element 49 of the second range clutch 28 is connected to a sungear 44 of the second summation planetary gear train 6 and to a planetcarrier 37 of the first summation planetary gear train 5.

Compared to the transmission 100 of FIG. 1, the two hydrostatic units Aand B of the transmission 100 FIG. 1 have the same sense of rotation inthe starting range, as is apparent from the speed diagram of FIG. 4. Thebridging of the expanded speed range or transmission-ratio range to thetransmission-ratio point X or of the starting process in this expandedspeed range corresponds to the bridging accomplished by the transmission100 shown in FIG. 1. The mechanical power is transmitted from the inputshaft 2 via the inlet shaft 34 of the first hydrostatic unit A and tothe first shaft of the summation gear train which comprises the internalgear 38 of planetary gear train 5. Hydrostatic power is transmitted viathe shaft 35 to the sun gear 36 of the first summation planetary geartrain 5. In the first operating range, the hydrostatic powersimultaneously divides into the first summation planetary gear train 5via sun gear 36 and into the second summation planetary gear train 6 viathe sun gear 39. A portion of the hydrostatic power is then summated viainternal gears 41 and 42 with the summed power at sun gear 44 derivedfrom the first summation planetary gear train 5 and transmitted to theplate 43 of the second summation planetary gear train 6. In the firstgear-shift range, the power is transmitted via clutch element 48 to theoutput shaft 3. In the reverse range, clutch or brake 26 is closed andthe power is transmitted, via the last planetary gear train stage 10 tothe sun gear 45, the planet gears on carrier 47 and the internal gear 46to the output shaft 3.

In terms of operation and design, the FIG. 5 is substantially the sameas the transmission 200 of FIG. 3. Transmission 300 is distinguished bythe presence of an additional third gear-shift area, so that the powervolume or the torque-changing range of the transmission, withoutchanging the size of the hydrostatic transmission, is almost doubled.The third range is realized by designing the first summation planetarygear train 7 as a four-shaft planetary gear train, preferably with anadditional planetary gear train step comprising sun gear 50, planetcarrier 51 and internal gear 52, and by connecting the fourth shaft,which comprises the sun gear 50, to an element 53 of the third rangeclutch 32. The speed curve of the hydrostatic units A and B, as well asof the output shaft 3, is shown in FIG. 6.

As a result of the expansion of the first range of in this way, there isobtained, aside from a substantial increase in the marginal power of thetransmission, and without increasing the installation space, startingcharacteristics that are gradual and without shock to satisfy thedriver's desires.

In these types of transmissions, the appropriate control signals areavailable anyhow, so the costs for the starting mechanisms arerelatively low.

In a specific embodiment not shown in the drawings, the by-pass valve 12is replaced by a pressure actuated valve. Here, one of the clutches 23,24, 26 or 27 may be additionally used as a starting clutch. This meansthat the transmission-ratio range up to the transmission-ratio point Xmust be bridged by the corresponding clutch by applying to thisclutch--as is customary with starting clutches--an appropriatelymetered, continuous pressure. The signals required for the continuouspressure buildup of the starting clutch are identical to the signalsmentioned above.

In order to close the clutch used as the starting clutch, a continuousclutch pressure is produced to ensure a gradual start without shock.This pressure is monitored by a controllable pressure valve, which iscontrolled by an appropriate signal. In the specific embodiment, thissignal is a signal which is dependent on brake actuation and/orengine-speed and/or load.

SEQUENCE OF OPERATIONS

By applying the brake, the direction of travel is preselected, as iscustomary in vehicles with automatic transmissions. Upon release of thebrake, the controllable pressure valve is already triggered by a signalwhich is dependent on the braking system in order to produce a clutchpressure so high that a minimum force arises at the wheels of thevehicle without pushing the engine below a minimum idling speed.Depending on the requirements, one can, by means of an additionalsignal, e.g., from the engine speed, influence the pressure controlvalve further by an additional signal, e.g., from the engine speed, inorder to further increase the clutch pressure or to adapt same to theprevailing conditions. For example, it is possible, by means of anautomatic control device, to exert an automatic influence on the enginecontrol, so that the engine cannot be stalled without stepping on thegas pedal. The clutch pressure can also be controlled, solely oradditionally, by a load-dependent signal, e.g. via the high pressure ofthe hydrostatic unit.

By means of this device it is possible, in accordance withvehicle-specific conditions, to create a smooth, sensitive andvehicle-oriented starting gear, which, when compared to prior artdevices, offers great advantages in terms of cost and installationspace.

I claim:
 1. A multiple path, infinitely variable hydromechanicaltransmission comprising:a rotatable power input shaft; a firstadjustable volume hydrostatic unit that is coupled to said power inputshaft for receiving input power therefrom; a second hydrostatic unithaving a hydrostatic power outlet shaft, said first and secondhydrostatic units being operably hydraulically coupled by high and lowpressure lines; a multiple shaft summation gear train having a firstshaft coupled to said power input shaft, a second shaft coupled to saidhydrostatic power outlet shaft, and a third shaft that may be coupled toa load, said gear train being arranged such that said second shaft isrotated by the gear train at a higher rotational speed than said powerinput shaft to thereby drive the second hydrostatic unit at a higherspeed than the first hydrostatic unit when the third shaft is coupled toa load at zero speed in a starting range; and a bypass valve operablyconnected between said high and low pressure lines for relievingpressure developed as a result of said second hydrostatic unit beingdriven by the gear train at a higher speed than the first hydrostaticunit.
 2. A transmission as set forth in claim 1, wherein said summationgear train includes a four-shaft planetary gear train and a three-shaftplanetary gear train collocated to the four-shaft planetary gear train,said three-shaft planetary gear train having a sun gear and an internalgear, said transmission further including a clutch for coupling saidhollow gear to said load, said gear trains being arranged so that thehydrostatic units rotate in opposite directions when the transmission isin said starting range.
 3. A transmission as set forth in claim 1,wherein said summation gear train includes at least two summationplanetary gear trains and a third gear train having an output shaft,said transmission including a clutch for coupling said output shaft to aload and at least two driving range clutches, said gear trains beingarranged so that the hydrostatic units rotate in the same direction whenthe transmission is in said starting range.
 4. A transmission as setforth in claim 1, wherein said summation gear train includes a firstfour-shaft summation planetary gear train that is collocated to saidhydrostatic units and a second summation planetary gear train, saidtransmission including at least three driving range clutches, a thirdthree-shaft planetary gear train and a reverse range clutch fordrivingly coupling said third planetary gear train into the powertransmission paths, said gear trains being arranged so that thehydrostatic units rotate in the same direction when the transmission isin said starting range.
 5. A transmission as set forth in claim 4,wherein said summation gear train includes a first four-shaft summationplanetary gear train that is collocated to said hydrostatic units and asecond summation planetary gear train, said transmission including atleast three driving range clutches, a third three-shaft planetary geartrain and a reverse range clutch for drivingly coupling said thirdplanetary gear train into the power transmission paths, said gear trainsbeing arranged so that the hydrostatic units rotate in the samedirection when the transmission is in said starting range.
 6. A multiplepath, infinitely variable hydromechanical transmission comprising:arotatable power input shaft; a first adjustable volume hydrostatic unitthat is coupled to said power input shaft for receiving input powertherefrom; a second hydrostatic unit having a hydrostatic power outletshaft; a multiple shaft summation gear train having a first shaftcoupled to said power input shaft, a second shaft coupled to saidhydrostatic power outlet shaft, and a third shaft that may be coupled toa load, said gear train being arranged such that said third shaft isrotated by the gear train at a rotational speed which corresponds to atransmission ratio point (x) when the third shaft is not coupled to aload at zero speed in a starting range; and a friction clutch forselectively coupling the third shaft to said load, and which serves bothas a start up clutch and as a driving range clutch.
 7. A transmission asset forth in claim 6, wherein said summation gear train includes afour-shaft planetary gear train and a three shaft planetary gear traincollocated to the four-shaft planetary gear train, said three-shaftplanetary gear train having a sun gear and an internal gear, saidinternal gear being connected to the third shaft of the multiple shaftsummation gear train, said gear trains being arranged so that thehydrostatic units rotate in opposite directions when the transmission isin said starting range.
 8. A transmission as set forth in claim 6,wherein said summation gear train includes at least two summationplanetary gear trains and a third gear train having an output shaft,said output shaft being connected to said third shaft of the multipleshaft summation gear train, said transmission including at least twoadditional driving range clutches, said gear trains being arranged sothat the hydrostatic units rotate in the same direction when thetransmission is in said starting range.
 9. A transmission as set forthin claim 6, wherein said clutch is controlled by a signal.
 10. Atransmission as set forth in claim 9, wherein said signal is a brakesignal, an engine RPM signal or a load-dependent signal.
 11. Atransmission as set forth in claim 10, wherein said signal is generatedby releasing a brake pedal.